Magnetic properties of iron marcasite FeS2

The magnetic susceptibility, χ, of a natural single crystal of marcasite, FeS₂, has been measured between 300K down to 4K. At room temperature χ=0.3×10^?5 emu/g and it is temperature independent down to 10K. Below 10K it increases up to 1.3×10^?5 emu/g. It is concluded that iron in marcasite is in the Fe²⁺ low spin state, and that the 6d electrons occupy the t_2g ground state. Consequently iron in marcasite (FeS₂) is not magnetic in agreement with our Mössbauer spectra recorded at 4.2K in an external magnetic field up to 39.9 kOe. The small value of χ is explained in terms of contributions from ppm impurities. i.e., diamagnetism and Van Vleck paramagnetism.     

Mössbauer investigation of iron disulphide (marcasite)

Mössbauer absorption of Fe⁵⁷ in naturally occuring and synthetic crystals of FeS₂, marcasite, has been studied in the ideal absorber thickness in the transmission geometry from a Co⁵⁷/Rh and Co⁵⁷/LiNbO₃ source. The recoilles fraction at room temperature, 298 K, has been obtained to be 0.2 and the mean square displacement < r² > was found to be 13×10^-19 cm² at room temperature, 298 K. Because of the small size of marcasite crystals it has not been possible to record good spectra of the monocrystals as a function of the orientation of the incident gamma rays from Co⁵⁷/LiNbO₃ source.     

Sulphur L2,3 and L1 XANES investigations of pyrite and marcasite

S L_2,3 and L₁ XANES of the polymorph minerals pyrite (FeS₂) and marcasite (two different cleavage planes) were measured in total electron yield mode using synchrotron radiation. Small but distinct differences were found in the fine structure of these spectra. Calculations with the FEFF-8 code were performed to reproduce the experimental data. Ex situ UV–ozone oxidation of the samples reveals different reactivity. Sulphate was identified on the pyrite and one marcasite cleavage planes whereas the second plane normal to the first one oxidises with changing quanta of sulphate and sulphite.     

Electrochemical performance of surfactant-processed LiFePO4 as a cathode material for lithium-ion rechargeable batteries

This study focuses on the effect of addition of surfactant as a dispersing agent during vibratory ball milling of LiFePO₄ (LFP) precursor materials on the electrochemical performance of solid-state reaction synthesized LFP for lithium-ion battery cathode material. LFP particles formed after calcinations of ball milled LFP precursors (Li₂CO₃, FeC₂O₄, and NH₄H₂PO₄) showed better size uniformity, morphology control, and reduced particle size when anionic surfactant (Avanel S-150) was used. The specific surface area of LFP particles increased by approximately twofold on addition of surfactant during milling. These particles showed significantly enhanced cyclic performance during charge/discharge due to a reduced polarization of electrode material. Electrodes fabricated from LFP particles by conventional milling process showed a 22 % decrease in capacity after 50 cycles, whereas the performance of electrode prepared by surfactant processed LFP showed only 3 % loss in capacity. The LFP particles were characterized using XRD, FE-SEM, particle size distribution, density measurement, and BET-specific surface area measurement. Electrochemical impedance spectra and galvanostatic charge/discharge test were performed for the electrochemical performance using coin-type cell.     

Physicochemical properties of core/shell structured pyrite FeS2/anatase TiO2 composites and their photocatalytic hydrogen production performances

A new photocatalytic system using anatase TiO₂ loaded onto pyrite FeS₂ (FeS₂/TiO₂) was developed to enhance the production of hydrogen. The FeS₂ (3.0, 5.0, 10.0, and 15.0 wt-%)/TiO₂ particles in SEM photos showed a core/shell structure composed of pyrite FeS₂ with a grape-like morphology of length of ～1.0 μm and anatase TiO₂ of diameter <50 nm. The evolution of H₂ by methanol/water (1:1) photo splitting over FeS₂/TiO₂ in a liquid system was enhanced as compared with that obtained using pure TiO₂ and FeS₂. In particular, 9.8 mmol of H₂ gas was produced in 10 h when 0.5 g of a 10.0 wt-% FeS₂/TiO₂ core/shell composite was used. Hydrogen production was increased by adding KOH electrolyte to 11.2 mmol. On the basis of cyclic voltammetry (CV) and UV–visible spectra results, this photoactivity of the FeS₂/TiO₂ composite was attributed to a shorter band gap than those of pure TiO₂ and FeS₂.     

Lithium ion conductive glass and its application to solid state batteries

Three kinds of coin-type battery, In-Li_x / Li_1−xCoO₂, Li_4/3+xTi_5/3O₄ / Li_1−xCoO₂, and Li_2+xFeS₂ / Li_1−xCoO₂, were fabricated with a Li⁺ ion conductive glass as an electrolyte, and their properties were investigated. They show excellent performance thanks to the solid electrolyte. Iron sulfide is found to be an excellent electrode material in solid state rechargeable batteries. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Comprehensive evolution mechanism of SOx formation during pyrite oxidation

Pyrite (FeS₂) oxidation during coal combustion is one of the main sources of harmful SO₂ emission from coal-fired power plants. Density functional theory (DFT) study was performed to uncover the evolution mechanism of SO_x formation during pyrite oxidation. The results show that chemisorption mechanism is responsible for O₂, SO₂ and SO₃ adsorption on FeS₂ surface. The presence of formed oxidation layer (Fe₂O₃) weakens the interaction between O₂ molecule and FeS₂ surface. The adsorbed O₂ molecule easily dissociates into active surface O atom for SO_x formation. The dissociation reaction of O₂ is activated by 77.38?kJ/mol, and exothermic by 138.46?kJ/mol. Compared to the further oxidation of SO₂ into SO₃, SO₂ prefers to desorb from FeS₂ surface. The dominant reaction pathway of SO₂ formation from the oxidation of the outermost FeS₂ surface layer is a three-step process: surface lattice S oxidation, SO₂ desorption and replenishment of S vacancy by activated surface O atom. The elementary reaction of surface lattice S oxidation has an activation energy barrier of 197.96?kJ/mol, and is identified as the rate-limiting step. SO₂ formation from the further oxidation of bulk FeS₂ layer is controlled by a four-step process: bulk lattice S migration, lattice S oxidation, SO₂ desorption and surface O atom deposition. Migration of lattice S from bulk position to the outermost surface shows a high activation energy barrier of 175.83?kJ/mol. The deposition process of surface O atom is a relatively easy step, and is activated by 21.05?kJ/mol.     

Combined magnetic and structural investigation of nano crystalline iron oxides: The interplay between crystal size and phase composition during FeS2 oxidation

The oxides that form during thermal oxidation of natural FeS₂ (pyrite and marcasite) consist of nanometer-sized crystals of α-Fe₂O₃ and γ-Fe₂O₃. This is shown with heating experiments that were made up to 650 °C, which resembles temperatures used in metallurgical processes. It is shown that magnetic measurements can play a key role in the investigation of this reaction, due to the unwanted blurring effects associated with finite crystal sizes if other methods are used. According to Mössbauer spectra combined with pXRD, many α-Fe₂O₃ crystals are in a stable magnetic state only due to the formation of bridging superexchange interactions in between them, but the γ-Fe₂O₃ experiences super-paramagnetic relaxation ceasing first at 20 K. Magnetisation measurements were used for two main purposes (1) determination of the amounts of γ-Fe₂O₃ in the products, and (2) for characterization of γ-Fe₂O₃ with respect to crystal size and possible magnetic surface effects such as spin-glass. It is proven that fine FeS₂ grains produce more γ-Fe₂O₃ than coarse. At 500 °C the fine FeS₂ grains oxidised into c. 30% γ-Fe₂O₃ and ca. 70% α-Fe₂O₃. At 525 °C, the γ-Fe₂O₃ amounts were also estimated in coarse oxidised FeS₂, and results were ca. 20% and 10% γ-Fe₂O₃ for the fine and coarse FeS₂ respectively. The γ-Fe₂O₃ crystal sizes were a function of both temperature and grain size, and it decreased with decreasing grain size, and upon rising the temperature from 450 to 550 °C. It is argued that the estimated errors during γ-Fe₂O₃ amount determination are due mainly to disordered magnetic sublattices at the crystal faces of γ-Fe₂O₃, giving an error of ca. 15% for those samples that have the smallest crystals.     

Synthesis of optimized LiNiO2 for lithium ion batteries

The synthesis of LiNiO₂, an attractive 4 V lithium ion battery cathode material, was investigated in view of identifying optimum preparation conditions by adopting various methods and comparing the structural, physical and electrochemical properties of the products. The conventional high temperature method (solid state annealing at 800 °C) and a novel low temperature method (self propagating high temperature method at 300 °C) allowed to synthesise crystalline LiNiO₂ with a composition close to the ideal stoichiometry. Optimisation of the preparation conditions which are responsible for forming high performance LiNiO₂ favoured LiNO₃ and Ni(NO₃)₂ as starting material along with an internal fuel (glycine) and a temperature of 300 °C for about three hours as most suitable heat treating condition. The electrochemical performance of LiNiO₂ synthesized via the various methods is reported.     

Effect of mesoporous carbon containing binary conductive additives in lithium ion batteries on the electrochemical performance of the LiCoO2 composite cathodes

Binary conductive additives (BCA), formed by sonication of mesoporous carbon (MC) and acetylene black (AB), were used as conductive additives to improve the electrochemical performance of a LiCoO₂ composite cathode. The electrochemical performance of the LiCoO₂ composite cathode dispersed with BCA was investigated. The results showed that the electrochemical performance (including the discharge capacity, the discharge voltage and the total internal resistance) of a BCA loaded LiCoO₂ composite cathode was better than that of a cathode loaded with AB. The possible mechanism is that the MC in BCA can adsorb and retain electrolyte solution, which allows an intimate contact between the lithium ions and the cathode active material LiCoO₂ due to its large mesopore specific surface area. A simplified model was also proposed.     

Enhanced electrochemical performance of lithium ion battery cathode Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>/C

Li-ion battery cathode material lithium-vanadium-phosphate Li₃V₂(PO₄)₃ was synthesized by a carbon-thermal reduction method, using stearic acid, LiH₂PO₄, and V₂O₅ as raw materials. And stearic acid acted as reductant, carbon source, and surface active agent. The effect of its content on the crystal structure and electrochemical performance of Li₃V₂(PO₄)₃/C were characterized by XRD and electrochemical performance testing, respectively. The results showed that the content of carbon source has no significant effect on the crystal structure of lithium vanadium phosphate. Lihtium vanadium phosphate obtained with 12.3% stearic acid demonstrated the best electrochemical properties with a typical discharge capacity of 119.4 mAh/g at 0.1 C and capacity retention behavior of 98.5% after 50 cycles. And it has high reversible discharge capacity of 83 mAh/g at 5 C with the voltage window of 3 to 4.3 V.     

Preparation of V-LiFePO4 cathode material for Li-ion batteries

The preparation of vanadium-modified olivine LiFePO₄ was attempted using vanadium-modified FePO₄ precursor which was synthesized by controlled crystallization. The structure and electrochemical behavior of V-LiFePO₄ with different vanadium contents were investigated. The electrochemical behavior of V-LiFePO₄ materials at high rate and low temperature was compared with that of the LiFePO₄ material. Incorporation of vanadium improved the electrochemical performance of LiFePO₄. The investigation showed that the 3%V-modified LiFePO₄ presented the best electrochemical performance.     

Preparation, characterization and electrochemical performance of γ-MnO2 and LiMn2O4 as cathodes for lithium batteries

The spinel LiMn₂O₄ is a very promising cathode material with economical and environmental advantages. LiMn₂O₄ materials have been synthesized by solid state method using γ-MnO₂ as manganese source, and Li₂CO₃ or LiNO₃ as Li sources. γ-MnO₂ is a commercial battery grade electrolytic manganese dioxide (TOSOH-Hellas GH-S) and LiMn₂O₄ samples were synthesized at a calcinations temperature up to 800 °C. γ-MnO₂ and LiMn₂O₄ samples were characterized by X-ray diffraction, thermal and electrochemical measurements. X-ray powder diffraction of as prepared LiMn₂O₄ showed a well-defined highly pure spinel single phase. The electrochemical performance of LiMn₂O₄ and its starting material γ-MnO₂ was evaluated through cyclic voltammetry, galvanostatic (constant current charge-discharge cycling) The electrochemical properties in terms of cycle performance were also discussed. γ-MnO₂ showed fairly high initial capacity of about 200 mAhg⁻¹ but poor cycle performance. LiMn₂O₄ samples showed fairly low initial capacity but good cycle performance.     

Lithium-rich layered nickel–manganese oxides as high-performance cathode materials: the effects of composition and PEG on performance

Lithium-rich layered nickel–manganese oxide (LRL-NMO) as a cathode material for rechargeable lithium-ion batteries was successfully prepared using an oxalic acid co-precipitation method, with polyethylene glycol (PEG1000) as an additive. The effects of the Ni/Mn ratio and of PEG on the phase purity, morphology, and electrochemical performance of LRL-NMO were investigated with X-ray diffraction, scanning electron microscope, electrochemical impedance spectroscopy, and charge/discharge testing. Li[Li_0.167Ni_0.25Mn_0.580]O₂ delivered the best electrochemical performance among the various Li[Li_1/3?2x/3Ni _x Mn_2/3?x/3]O₂ (0?<?x?<?0.5) materials. Furthermore, the sample to which an appropriate amount of PEG had been added showed much smaller and more uniform particle size, higher discharge capacity and energy density, better cycling stability, and lower resistance. The material prepared by adding 9 wt% PEG exhibited high discharge capacity and stability; after 100 cycles at 2 C, it still delivered a discharge capacity of 125.6 mAh g^?1, which was 50 % higher than that of a sample prepared without PEG.     

Study on electrochemical performance of Mg-doped Li<Subscript>2</Subscript>FeSiO<Subscript>4</Subscript> cathode material for Li-ion batteries

In this paper, Li₂Fe_1?yMg_ySiO₄/C (y?=?0, 0.01, 0.02, 0.03, 0.05), a cathode material for lithium-ion battery was synthesized by solid-state method and modified by doping Mg²⁺ on the iron site. The effects of Mg²⁺ doping on the crystal structure and electrochemical performance Li₂FeSiO₄ was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical tests. Electrochemical methods of measurement were applied including constant current charge–discharge test, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), to determine the electrochemical performance of the material and the optimal doping ion and ratio. The results showed that Li₂Fe_0.98Mg_0.02SiO₄/C has the higher specific capacity and better cycle stability as well as lower impedance and better reversibility. The enhanced electrochemical performance can be attributed to the increased electronic conductivity, the decreased charge transfer impedance, and the improved Li-ion diffusion coefficient. Then, further study on the synthesis conditions was performed to find the optimal combustion temperature and time. According to the study, the material which has the best electrochemical performance, shows initial discharge specific capacity of 142.3 mAh g^?1 at 0.1 C (1 C?=?166 mA g^?1) and coulomb efficiency of 95.6%, under the condition that the temperature is 700 °C and the calcining time is 10 h.     

Effect of cooling method on the electrochemical performance of 0.5Li2MnO3·0.5LiNi0.5Mn0.5O2 cathodes

0.5Li₂MnO₃·0.5LiNi_0.5Mn_0.5O₂ powders were synthesized by coprecipitation, and high temperature sintered with different cooled methods, such as cooled with furnace material, water material, and in liquid nitrogen (N-material). The effect of cooling methods on physical and electrochemical properties are discussed through the characterizations of X-ray diffraction (XRD), scanning electron microscopy, electrochemical impedance spectroscopy (EIS), and discharge, cyclic, and rate tests. XRD results show that all samples exhibit layered characteristics. The electrochemical performance results indicate that the N-material has the best electrochemical performance. The discharge capacity at 0.1 and 5 C are 279 and 99 mAhg^?1, respectively. The coulomb efficiency is highest, 78.4 %. The capacity retention after 50 cycles at 0.2 C is 97.1 %. EIS results show that the charge transfer resistance of N-materials is lowest, which is responsible for higher rate capacity.     

Low temperature synthesis of layered LiNiO2 cathode material in air atmosphere by ion exchange reaction

The layered LiNiO₂ cathode material for lithium ion battery was synthesized by ion-exchange reaction at low temperature in air atmosphere. The influence of synthesis conditions on the electrochemical performance of the resulting LiNiO₂ was investigated. The LiNiO₂ samples were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM) and infrared (IR) analysis. The results indicate that low temperature fabricated LiNiO₂ powders keep a single layered hexagonal structure and homogenous spheric shape like the raw material NiOOH. Charge and discharge tests show that the resultant LiNiO₂ exhibits good electrochemical properties. The first charge and discharge capacities of the sample are 183.4 mA h g^− 1 and 169.5 mA h g^− 1 at 0.5 mA cm^− 2, respectively. Galvanic charge/discharge and cyclic voltammetry tests reflect that LiNiO₂ electrode exhibits good cycle reversibility.     

Synthesis and structure refinement studies of LiNiVO4 electrode material for lithium rechargeable batteries

The lithium nickel vanadate (LiNiVO₄) cathode material has been synthesized by using sol-gel method. The thermal behavior of the material has been examined by thermogravimetric and differential thermal analysis (TG/DTA). The structure of LiNiVO₄ compound has been studied by the Rietveld refined x-ray diffraction (XRD) technique. The Brunauer–Emmett–Teller (BET) surface area of 0.79 m² g^?1 was estimated with N₂ absorption characteristics. The synthesized powder morphology was observed by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) studies of synthesized LiNiVO₄ powder indicate that the oxidation states of nickel and vanadate are +2 and +5, respectively. The electrochemical properties were monitored using 2032 coin cells by cyclic voltammetry and EIS, which showed that the microscopic structural features were deeply related with the electrochemical performance.     

Enhanced lithium storage performance of a self-assembled hierarchical porous Co<Subscript>3</Subscript>O<Subscript>4</Subscript>/VGCF hybrid high-capacity anode material for lithium-ion batteries

A Co₃O₄/vapor-grown carbon fiber (VGCF) hybrid material is prepared by a facile approach, namely, via liquid-phase carbonate precipitation followed by thermal decomposition of the precipitate at 380 °C for 2 h in argon gas flow. The material is characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Brunauer-Emmett-Teller specific surface area analysis, and carbon elemental analysis. The Co₃O₄ in the hybrid material exhibits the morphology of porous submicron secondary particles which are self assembled from enormous cubic-phase crystalline Co₃O₄ nanograins. The electrochemical performance of the hybrid as a high-capacity conversion-type anode material for lithium-ion batteries is investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic discharge/charge methods. The hybrid material demonstrates high specific capacity, good rate capability, and good long-term cyclability, which are far superior to those of the pristine Co₃O₄ material prepared under similar conditions. For example, the reversible charge capacities of the hybrid can reach 1100–1150 mAh g^?1 at a lower current density of 0.1 or 0.2 A g^?1 and remain 600 mAh g^?1 at the high current density of 5 A g^?1. After 300 cycles at 0.5 A g^?1, a high charge capacity of 850 mAh g^?1 is retained. The enhanced electrochemical performance is attributed to the incorporated VGCFs as well as the porous structure and the smaller nanograins of the Co₃O₄ active material.     

Investigation of Virgin Coals and Coals Subjected to a Mild Acid Treatment

A quantitative determination of the relative marcasite/pyrite contents in virgin coals is possible by means of ⁵⁷Fe Mössbauer spectroscopy. Complications arise, however, when iron-containing silicates, carbonates, or other salts are present. The application of a mild chemical treatment involving hydrofluoric acid has been employed to remove these Fe-containing phases while leaving the iron-disulfide phases unaffected. Several South African coal samples with non-iron disulfide, Fe-containing phases ranging from 18 to 30 weight percent have been subjected to a hydrofluoric acid leaching at room temperature. The loss of mineral matter with HF leaching correlates well with the mineral matter residue following low temperature ashing. The ⁵⁷Fe Mössbauer spectra of the resulting coal samples, collected at 297K, indicate that only FeS₂ phases are present. The Mössbauer parameters for these samples (0.619 ≤E_Q≤0.622 mm s^?1; 0.306≤δ≤0.309 mm s^?1) indicate the absence of appreciable quantities of marcasite in the coals. These Mössbauer parameters differ slightly, but systematically, from those of pyrite for which a quadrupole splitting of 0.6110 ± 0.0020 mm s^?1 has been established. On the basis of previous studies, these increased E_Q values suggest the presence of As substitution in the pyrite phases. ⁵⁷Fe Mössbauer spectra of virgin coals exhibit phase assemblages comparable to those observed by x-ray diffraction (XRD), e.g. pyrite and (Ca,Fe)CO₃, even though the presence of pyrite is less definite in the XRD data.